One of the jobs roots do is gather nutrients from soils. When they’re in soils where the nutrient levels are patchy, they target roots at the richest soils. This behaviour is called foraging precision. But how does the plant work out where these patches are?
Akira Yamawo and colleagues wanted to find out, and tested an idea called shoot-root signalling. It’s thought that signals from the leaves may work back to the roots to give feedback on where to forage. The team thought, if this were correct, that damage to leaf veins – but not nearby tissues – would affect foraging precision.
What they did was test the idea on two plants Plantago asiatica and Prunus jamasakura. In some of the plants, they took off the tip of the main veins in the leaves. In others, they damaged the leaves in the mesophyll between the veins.
The results were striking. Where the mesophyll was damaged, there was more root biomass in the nitrogen-rich patches than the nitrogen-poor patches. This activity was business as usual. However, when the veins were damaged, neither plant increased biomass in nitrogen-rich patches. The distribution of roots was pretty much uniform.
The authors say, “Our results show that root foraging precision involves the whole plant, not only root tissues but also shoot tissues.” They then discuss the implications further. When an insect is eating a leaf, what sort of damage is it doing? If it’s eating mesophyll, then it’s reducing the plant’s ability to photosynthesise. But if it’s going for the veins, then Yamamo and colleagues say it will also impact root foraging precision. The attack becomes a double-hit on the plant.
“It is surprising that these effects were found even when damage was relatively minor,” the authors write. “For example, we imposed damage to only the five youngest leaves of P. jamasakura, yet still found large effects on root foraging precision, even though they had about 50 leaves.” So these results could show that a herbivore doesn’t have to damage the majority of leaves to give the plant a problem.
This susceptibility to damage might explain why some plants defend young leaves more heavily than older leaves. It becomes necessary to protect shoot-root signalling. It also suggests, the authors say, that future work in plant herbivory should also examine shoot-root signalling and plant behaviour.